Add compile instructions to README/Makefile

[DivineDominion]

I'm on a M1 (arch arm64) Mac and cannot load the x86_64 .dylib. I am not versed in Rust development, so I tried to re-build the .dylib by running cargo build --release from the core/ subdirectory.

Possible steps for a Makefile:

cd core; cargo build --release
cp core/target/release/libflx_rs_core.dylib ./bin/

[DivineDominion]

Making a universal binary, i.e. arm64 and x86_64 is a WIP it seems: rust-lang/cargo#8875

This repo or https://github.com/jcs090218/flx-rs could maybe use GitHub actions instead to prepare a universal binary and attach that to releases. Until then, compiling locally is the only option for users, so a makefile step would be the cheapest solution

[jcs090218]

Thanks for the information! ❤️

This repo already uses GitHub actions to build binary files, see .github/workflows/build.yml. Of course this isn't universal, but we could wait until Rust has made those changes. :)

[jcs090218]

@DivineDominion After #11, you should able to read flx-rs.aarch64-apple-darwin.dylib file.